The Hemi has more potential in the long run (in my opinion at least), which is one of the main reasons I went with one over an older style engine. I was debating a small block stroker build, a supercharged junkyard Magnum build, an aluminum big block build, and a modern hemi swap and they all had similar starting prices. You had to build the smallblocks up quite a bit though and they became pretty high strung to get the same kind of power a modern Hemi could get with far less parts. My mileage has slowly creeped up as I've worked on the tune and other stuff, but in reality I'm still pushing a brick through the air so I can't expect too much. The mileage hasn't changed much from the 16mpg I was getting with the junkyard 318 that was in the car when I bought it, but I have probably doubled the horsepower and still get 16-18 mpg. Might be closing in on the magic 20 I've been aiming for with some better tuning.